Ferrite-based spheroidal graphite cast iron and exhaust system component using the same

ABSTRACT

The present invention provides a ferrite-based spheroidal graphite cast iron containing the following elements in the following contents in % by weight: C: 3.1 to 4.0%; Si: 3.6 to 4.6%; Mo: 0.3 to 1.0%; V: 0.1 to 1.0%; Mn: 0.15 to 1.6%; and Mg: 0.02 to 0.10%, and the total content of V and Mn is 0.3 to 2.0 wt %, and an exhaust system component using the spheroidal graphite cast iron. Thus, the ferrite-based spheroidal graphite cast iron having higher heat resistance than that of a conventional high Si spheroidal graphite cast iron that can be produced inexpensively by a simple method.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to high heat resistant spheroidal graphite cast iron obtained by improving a high Si spheroidal graphite cast iron conventionally used only at a temperature of 800° C. or less by an alloy design so as to be used at a high temperature of 850 to 900° C. The spheroidal graphite cast iron is a ferrite-based high Si spheroidal graphite cast iron and the raw material cost is lower and the castability and the machinability are better than those of competing stainless cast steel or Ni-resist cast iron. Therefore, the spheroidal graphite cast iron can be widely used for automobile exhaust system components such as exhaust manifolds, turbo housings, turbo housing-integrated exhaust manifolds, or turbo outlet pipes.

[0003] 2. Description of the Related Art

[0004] Environmental problems are becoming more and more severe, and for purposes of catalyst purification efficiency and lower fuel consumption the temperature of exhaust gas of automobiles is increasing. Under these circumstances, high heat resistant pipe exhaust manifolds employing stainless steel pipes, or sheet metal exhaust manifolds obtained by plastic working of a stainless steel sheet are starting to be used for the exhaust manifold of an engine.

[0005] In order to increase the purification efficiency of exhaust gas, it is necessary to pass a high temperature exhaust gas through a catalyst, and to dispose a heavy maniverter containing a catalyst as near the exhaust manifold as possible. In particular, a turbine housing provided with a turbine rotor is connected between the exhaust manifold and the maniverter in a turbo car, and therefore a load is increased in the exhaust manifold, requiring higher rigidity than that at high temperature.

[0006] The above-described pipe exhaust manifolds or sheet metal exhaust manifolds are easily deformed at high temperature because of linear expansion inherent to stainless steel and small thickness, they have a poor degree of freedom in the shape, and therefore cast iron still has to be used for exhaust manifolds for a turbo car at present. For conventional cast iron materials for exhaust manifolds, Mo-added high Si spheroidal graphite cast iron containing 3.6 to 4.0% of Si and 0.3 to 1.0% of Mo is only used in practice.

[0007] As a technique for improving the high temperature properties in the range of the exhaust gas temperature, for example, Japanese Patent Provisional Publication Nos. 4-218645, 5-125494, and 7-48653 disclose stainless cast steel, but there is no disclosure as to a cast iron having a content of C of 2.1 wt % or more.

[0008] Furthermore, as an example in which the properties of high Si spheroidal graphite cast iron are improved, there are techniques aiming at improving brittleness in a middle temperature range, as disclosed in Japanese Patent Provisional Publication Nos. 10-195587 and 61-73859.

[0009] However, there are the following problems. Although Japanese Patent Provisional Publication No. 61-73859 describes adjusting the composition of Mg and P, it is difficult to control the adjustment in an actual production line. In the method for producing the spheroidal graphite cast iron disclosed in Japanese Patent Provisional Publication No. 10-195587, arsenic (As), which is deadly toxic, is added, so that the work environment is very bad.

[0010] On the other hand, the conventionally used high Si spheroidal graphite cast iron has a low A_(c1) transformation point of about 850° C., at which a matrix of a ferrite and pearlite structure is transformed to an austenite phase by heating. Therefore, when the high Si spheroidal graphite cast iron is exposed to a high temperature exhaust gas (880 to 930° C.), the temperature of an exhaust system component itself is increased to 800 to 880° C. and exceeds the A_(c1) transformation point, so that the high Si spheroidal graphite cast iron is transformed readily to the austenite phase and thermal fatigue or deformation occurs because of rapid elongation increase and strength decrease. Therefore, when seeking for higher heat resistance than that of the conventional high Si spheroidal graphite cast iron, only Ni-resist cast iron and stainless cast steel are practical at present.

[0011] However, the Ni-resist cast iron and the stainless cast steel contain a large amount of Ni, Cr, W or the like for the raw material, so that the cost of the raw material is high. In addition, since the melting point of the raw material is high, production cannot be performed with a conventional cast iron production facility, and the castability, the production yield and the machinability are poor, so that the costs of components are significantly high.

SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to solve the above problem and to provide a ferrite-based spheroidal graphite cast iron that has a higher heat resistance than that of the conventional high Si spheroidal graphite cast iron and can be produced inexpensively by a simple method.

[0013] In order to achieve the above object, a ferrite-based spheroidal graphite cast iron of the present invention includes C, Si, Mo, V, Mn, and Mg, and the remaining portion is composed of Fe and inevitable impurities.

[0014] The ferrite-based spheroidal graphite cast iron of the present invention has excellent tensile strength and yield strength in a region from room temperature to the vicinity of 800 to 900° C. Therefore, when this spheroidal graphite cast iron is applied to an exhaust system component, for example, an exhaust manifold, the component can resist high temperature exhaust gas having a temperature around 880 to 930° C. sufficiently, and therefore the temperature of the exhaust gas can be increased. Thus, efficient purification of the exhaust gas and fuel saving can be achieved, and consequently the present invention can comply with coming exhaust gas regulations such as post, post 53 or the like (legislative regulations of Japan).

[0015] Furthermore, since in the ferrite-based spheroidal graphite cast iron of the present invention, casting methods and conditions of the conventional spheroidal graphite cast iron can be used without any change, the present invention can be produced in the existing cast iron production line, and new facility investment is not required. Furthermore, the raw material costs and the processing costs are lower than those of stainless cast steel and Ni-resist, so that the production costs can be low. In addition, unlike stainless cast steel and Ni-resist, the ferrite-based spheroidal graphite cast iron has excellent machinability and castability, so that the degree of freedom in the shape of an exhaust system component can be increased.

[0016] In another embodiment of the ferrite-based spheroidal graphite cast iron of the present invention, the contents of the above elements in % by weight are as follows: C: 3.1 to 4.0%; Si: 3.6 to 4.6%; Mo: 0.3 to 1.0%; Mg: 0.02 to 0.10%, V: 0.1 to 1.0%; and Mn: 0.15 to 1.6%.

[0017] The content of the inevitable impurities are as follows: S: 0.02% or less; and P: 0.1% or less, and the total content of Cu, Sn and Cr is 0.8% or less, preferably, 0.4% or less. An amount of 0.4% or less is preferable for the following reason. These elements, Cu, Sn and Cr, promote to precipitate pearlite, and therefore when the amount of these elements mixed is increased, the amount of the pearlite precipitated in the matrix is increased. Thus, the hardness is increased, which leads to a reduction in the elongation. Furthermore, when the amount of the pearlite precipitated is increased, more of cementite (Fe₃C) in the pearlite is dissolved at high temperature, and consequently graphite growth is facilitated, and the quality is deteriorated.

[0018] Furthermore, in still another embodiment of the ferrite-based spheroidal graphite cast iron of the present invention, the total content of V and Mn is 0.3 to 2.0 wt %.

[0019] Mn facilitates precipitation of the pearlite microstructure so as to contribute to improvement of tensile strength and yield strength, and V forms and precipitates fine carbides having a high melting point in the vicinity of the grain boundaries of eutectic cells so as to serve to improve the grain boundary potential and to prevent the pearlite microstructure from dissolving at high temperature. Therefore, the total amount of Mn and V is in the range from 0.3 to 2.0%, and the above-described effect can be larger by adding the two elements at the same time (so-called multiple addition).

[0020] Furthermore, in yet another embodiment of the ferrite-based spheroidal graphite cast iron of the present invention the Si/CE value is 0.97 or less.

[0021] This CE value is referred to as carbon equivalent and is given by the content of C+(the content of Si+the content of P)/3. When the Si/CE value is adjusted to 0.97 or less, a reduction in the elongation in the spheroidal graphite cast iron in a range from room temperature to middle temperature is suppressed, and the thermal fatigue resistance can be improved further. Mg is an element playing an important role as a graphite spheroidizing agent.

[0022] Furthermore, an exhaust system component of the present invention is produced using the ferrite-based spheroidal graphite cast iron.

[0023] In one embodiment of the exhaust system component, the exhaust system component can be an exhaust manifold, a turbo housing, a turbo housing-integrated exhaust manifold, or a turbo outlet pipe for automobiles.

[0024] Since the exhaust system component is made of a cast material, the degree of freedom in the shape is larger than that of, for example, pipe exhaust manifolds using stainless steel pipes, or sheet metal exhaust manifolds that are processed from stainless steel sheets. Therefore, a complex shaped component can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a graph showing the relationship between the Si content and the transformation temperature.

[0026]FIG. 2 is a graph showing the relationship between the test temperature (20 to 900° C.) and the tensile strength in examples.

[0027]FIG. 3 is an enlarged graph of FIG. 2 showing the relationship between the test temperature (700 to 900° C.) and the tensile strength.

[0028]FIG. 4 is a graph showing the relationship between the test temperature (20 to 900° C.) and the high temperature proportional limit in examples.

[0029]FIG. 5 is an enlarged graph of FIG. 4 showing the relationship between the test temperature (700 to 900° C.) and the high temperature proportional limit.

[0030]FIG. 6 is a graph showing the amount of Ni added and the tensile strength in examples.

[0031]FIG. 7 is a graph showing the amount of Mn added and the tensile strength in examples.

[0032]FIG. 8 is a graph showing the amount of V added and the tensile strength in examples.

[0033]FIG. 9 is a graph showing the relationship between the test temperature (20 to 900° C.) and the high temperature tensile strength in examples.

[0034]FIG. 10 is an enlarged graph of FIG. 9 showing the relationship between the test temperature (750 to 900° C.) and the high temperature tensile strength.

[0035]FIG. 11 is a graph showing the relationship between the test temperature (20 to 900° C.) and the elongation in examples.

[0036]FIG. 12 is a graph showing the relationship between the Si/CE value and the elongation in examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Hereinafter, ferrite-based spheroidal graphite cast iron of this embodiment of the present invention will be described in detail.

[0038] In order to solve the above-described problems, it is most advantageous to improve the conventionally used high Si spheroidal graphite cast iron containing Mo by an alloy design. The inventors of the present invention conducted in-depth research on the following items in order to improve the heat resistance of the high Si spheroidal graphite cast iron containing Mo and thus achieved the present invention. In the following description, “%” refers to “% by weight” in any cases.

[0039] 1. Increasing the A₁ transformation point

[0040] 2. Improving thermal deformation resistance

[0041] 3. Improving thermal fatigue resistance

[0042] 4. Improving oxidation resistance

[0043] These items will be described below.

[0044] 1. Increasing the A₁ Transformation Point

[0045] First, in order to improve the heat resistance of a ferrite-based spheroidal graphite cast iron, it is necessary to increase, in particular, the A_(c1) transformation point of the A₁ transformation points. The A_(c1) transformation point refers to the temperature at which the matrix structure in which ferrite and pearlite are mixed is transformed to an austenite phase by heating. Therefore, when the A₁ transformation point is increased, the spheroidal graphite cast iron is hardly transformed to the austenite phase, giving improved heat resistance. The A₁ transformation point is increased, as the amount of Si is increased, so that Si is added in an amount more than that of a conventional cast iron material as far as practically possible, with the lower limit is set to 3.6%. However, when Si is added excessively in an amount of more than 4.6%, a significant reduction in elongation occurs in the spheroidal graphite cast iron, so that the upper limit is set to 4.6%. Therefore, the amount of Si added is 3.6% to 4.6%, preferably 4.0% to 4.5%.

[0046] Thus, the A_(c1) transformation point can be increased to about 890° C. by adding about 4.5% of Si, whereas the A_(c1) transformation point is about 850° C. in the conventional cast iron.

[0047] In general, the temperature of an exhaust system component exposed to a high temperature exhaust gas of 880° C. to 930° C. is increased to the vicinity of 800 to 880° C. Therefore, when the ferrite-based spheroidal graphite cast iron of the present invention is applied to an exhaust system component, the temperature does not exceed the A_(c1) transformation point even during engine operation, so that a large transformation strain involved in phase transformation can be suppressed from occurring, and the thermal fatigue life can be improved significantly.

[0048] 2. Improvement of Thermal Deformation Resistance

[0049] In order to suppress the thermal deformation that is caused by heating or cooling when elongation or contraction is constrained, it is advantageous to improve the high temperature strength, in particular, the high temperature yield strength or the high temperature proportional limit.

[0050] Therefore, in order to improve the strength of the ferrite-based spheroidal graphite cast iron at high temperature, it is advantageous to add V and Mn to the Si and Mo-based spheroidal graphite cast iron.

[0051] When the ferrite-based spheroidal graphite cast iron is used, for example, for an exhaust system component, in the vicinity of the upper limit (about 850° C.) of the temperature of the exhaust system component itself during engine operation, the larger the amount of Mn and Ni added, the greater the tensile strength. On the other hand, for V, an effect of adding V can be recognized at 0.1%, and when 0.3% or more of V is added, substantially constant tensile strength can be maintained.

[0052] Herein, Mn has an important effect of facilitating precipitation of pearlite so as to improve the tensile strength and the yield strength, so that the content of Mn is 0.15% or more. Furthermore, V forms fine carbide having a high melting point and precipitates it in the vicinity of the grain boundaries of eutectic cells so as to serve to improve the grain boundary potential and to prevent pearlite from dissolving at high temperature. Therefore, the content of V is 0.1% or more. The effects of Mn and V improve the strength from room temperature to high temperature.

[0053] On the other hand, when Mn is added in an amount of more than 1.6% and V is added in an amount of more than 1.0%, the pearlite ratio in the matrix of the spheroidal graphite cast iron becomes high and elongation is reduced in a room temperature range and a middle temperature range, so that it is not preferable to add Mn and V in amounts more than those. Therefore, the upper limit of the content of Mn is 1.6%, and the upper limit of the content of V is 1.0%.

[0054] Therefore, the content of Mn is 0.15 to 1.6%, preferably 0.15 to 1.5%. The content of V is 0.1 to 1.0%, preferably 0.2 to 0.5%.

[0055] As described above, in order to improve the high temperature properties of the spheroidal graphite cast iron, it is advantageous to add V and Mn. In addition, V and Mn can provide a more desirable effect to the mechanical properties or the like, when they are added in combination than when each of them is added alone, The total amount of Mn and V is 0.3 to 2.0%, preferably 0.4% to 1.8%.

[0056] Furthermore, Mo is also an element that improves the mechanical properties at high temperature, in particular, the high temperature yield strength (or high temperature proportional limit). When the content of Mo is less than 0.3%, the effect of addition is small, so that the lower limit of the content of Mo is 0.3%. On the other hand, although the Al transformation point does not depend on the content of Mo, when the content of Mo exceeds 1.0%, the pearlite ratio in the spheroidal graphite cast iron is increased and the hardness is increased, so that a significant reduction in elongation occurs. Therefore, the upper limit of the content of Mo is 1.0%.

[0057] Thus, the content of Mo is 0.3 to 1.0%, preferably 0.3% to 0.7%.

[0058] When Ni is added, the mechanical properties at high temperature can be improved by the addition, but the A₁ transformation point is reduced, so that Ni is not suitable for the spheroidal graphite cast iron of the present invention. The A₁ transformation point is reduced, because Ni is an element for stabilizing austenite and therefore reduces the A₁ transformation point. Furthermore, it is not confirmed that the above-described added elements prevent the spheroidal graphite cast iron from becoming spheroidal.

[0059] Summing up, the following component constitution can improve the high temperature properties of the spheroidal graphite cast iron of the present invention.

[0060] i) V and Mn are added to the ferrite-based spheroidal graphite cast containing 3.6 to 4.6% of Si and 0.3 to 1.0% of Mo that serves as the base.

[0061] ii) The amounts of V and Mn added are 0.1 to 1.0% and 0.15% to 1.6%, respectively, and the total amount of V and Mn added is 0.3 to 2.0 wt %, preferably 0.4 to 1.8%.

[0062] 3. Improvement of Thermal Fatigue Resistance

[0063] In the high Si spheroidal graphite cast iron containing Mo, the following two approaches are conceivable in order to enhance the thermal fatigue resistance: an approach for canceling reduction in elongation that occurs in the vicinity of 400 to 500° C. inherent to the spheroidal graphite cast iron; and an approach for improving the tensile strength or the yield strength from room temperature to high temperature. The techniques disclosed in Japanese Patent Provisional Publication Nos. 61-73859 and 10-195587 described in the section “Description of the Related Art” employ the former approach, whereas the present invention is based on the latter approach. More specifically, the present invention focuses on suppressing plastic deformation with respect to tensile strain generated in heating and cooling cycles by enhancing the yield strength (yield point or proportional limit) so as to increase the life, which is a period up to the time an initial crack occurs.

[0064] However, in the spheroidal graphite cast iron of the present invention, in order to further improve or stabilize the thermal fatigue characteristics, an approach for ensuring elongation from room temperature to middle temperatures (in the vicinity of 400 to 500° C.) has been examined. Basically, when elongation is small, the thermal fatigue life is reduced. This is because when elongation is small, the sensitivity to cracks with respect to tensile strain involved in the compression plastic deformation at high temperature becomes large in a range from room temperature to middle temperature. Then, as a result of examination of components for ensuring elongation without reducing the total amount and the composition ratio of V and Mn added, it was clarified that the elongation of the Mo-added high Si spheroidal graphite cast iron depends significantly on the mixing ratio of C and Si, that is, the Si/CE value (or C/CE value). Herein, the “CE value” refers to carbon equivalent and is calculated with an equation: the content of C+1/3 (the content of Si+the content of P).

[0065] The elongation of the Mo-added high Si spheroidal graphite cast iron tends to be reduced drastically when the Si/CE value is 0.97 or more. Therefore, the Si/CE value is at most 0.97, preferably 0.82 to 0.96. In this lower limit (0.82) of the Si/CE, value, the content of C is 3.5%, the content of Si is 4.0%, and the content of P is 0.06%. The upper limit (0.96) is set, based on the results shown in FIG. 7.

[0066] Furthermore, according to the range of the Si/CE value that is set as above, the lower limit of a preferable content of C is 3.1%, and the upper limit of Si is 4.5%. On the other hand, as the content of Si is increased, the solid solubility of C is reduced, and graphite floatation occurs in the spheroidal graphite cast iron, thus leading to a variation in the particle size of graphite and a reduction in the graphite nodule count. Therefore, the upper limit of the content of C is 4.0%. That is to say, the content of C is 3.1 to 4.0%, preferably 3.1 to 3.7%.

[0067] Furthermore, the total amount of V and Mn is 0.3 to 2.0%, as described above. This will be described briefly. It is important to limit the pearlite ratio in the spheroidal graphite cast iron to 40% or less by setting the upper limits of V and Mn to 1.0% and 1.6%, respectively, and the upper limit of the total amount to 2.0%. The lower limit of the total content of V and Mn is 0.3% for a sufficient effect of multiple addition.

[0068] Furthermore, the total amount of Cu, Sn and Cr, which are inevitable impurities and increase the amount of pearlite and thus improve the hardness and reduce the elongation, is limited to 0.8% or less, preferably 0.4% or less. An amount of 0.4% or less is preferable for the following reason. These elements promote to precipitate pearlite, and therefore when the amount of these elements mixed is increased, the amount of the pearlite precipitated in the matrix is increased. Thus, the hardness is increased, which leads to a reduction in the elongation. Furthermore, when the amount of the pearlite precipitated is increased, more of cementite (Fe₃C) in the pearlite is dissolved at high temperature, and consequently graphite growth is facilitated, and the quality is deteriorated.

[0069] An excessive mixture of S prevents graphite from being spheroidized and causes a reduction in the elongation, so that it is necessary that the content of S is 0.02% or less.

[0070] The content of Mg, which is a graphite spheroidizing agent, is 0.02 to 0.10%, preferably 0.02 to 0.06%. The content of P is 0.1% or less.

[0071] 4. Improvement of Oxidation Resistance

[0072] The oxidation resistance of the spheroidal graphite cast iron depends on the content of Si, so that the high Si spheroidal graphite cast iron material has a better oxidation resistance than that of regular cast iron materials. The amount of an oxide film produced on the surface of an exhaust system component or the like employing the spheroidal graphite cast iron is smaller, as the content of Si is larger. Consequently, through-cracks due to cracks in the oxide film are suppressed from occurring, which contributes to improvement of the life. Therefore, the amount of Si can be determined in the range described in the items 1 to 3.

[0073] The ferrite-based spheroidal graphite cast iron of the present invention can provide a product having better heat resistance than that of the conventionally used casting material containing a high concentration of Si and Mo. Since it is a cast iron material, the ferrite-based spheroidal graphite cast iron has excellent machinability and castability, unlike stainless cast steel or Ni-resist. Therefore, according to the present invention, for example, a high performance exhaust system component can be produced at a low cost.

EXAMPLES

[0074] The present invention will be described more specifically by way of example.

[0075] Various experiments were conducted, based on the following four approaches.

[0076] 1. Increasing the A₁ transformation point

[0077] 2. Improving the thermal deformation resistance

[0078] 3. Improving the thermal fatigue resistance

[0079] 4. Improving the oxidation resistance

[0080] 1. Increasing the A_(c1) Transformation Point

[0081] Examinations were conducted to increase the A₁ transformation point at which the texture structure in which ferrite and pearlite are mixed is transformed to the austenite phase by heating.

[0082] Table 1 shows the results of measuring the Si content and the transformation temperatures of samples used for transformation temperature measurement, and FIG. 1 is a graph showing these results. TABLE 1 Samples used for transformation temperature measurement and measurement results A_(c1) A_(c1) A_(r1) A_(r1) A₁ start A_(c1) end transformation start A_(r1) end transformation transformation Si temperature temperature point temperature temperature point point (wt %) (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) Comparative 4.0 861.2 898.7 880 836.0 798.9 817 849 material 1 (conventional cast iron) Comparative 3.6 852.3 902.5 877 831.8 789.1 810 844 material 2 (conventional cast iron) Comparative 4.0 869.5 909.3 889 846.1 801.2 824 857 material 3 (conventional cast iron) Test 4.4 897.4 918.6 908 864.3 820.0 842 875 material 1 Test 4.5 887.0 934.2 911 879.2 828.8 854 882 material 2

[0083] The A₁ transformation point is increased, as the Si amount is increased, as shown in FIG. 1 as well, so that it is preferable that the Si amount is larger than that of the conventionally used cast iron material and as large as possible in practice, for example, 3.6 to 4.6%. The highest A_(c1) transformation point of the conventional cast iron is 889° C. (comparative material 3), but when the Si amount is increased to 4.4 to 4.5%, the highest A_(c1) transformation point is increased to 908 to 911° C. (test materials 1 and 2). Thus, when the spheroidal graphite cast iron of the present invention is applied to an exhaust system component of automobiles, the metal temperature of the exhaust system component exposed to high temperature exhaust gas (880° C. to 930° C.) is lower than the A_(c1) transformation point. Therefore, since it hardly exceeds this transformation point even by heating or cooling during heavy engine operation, a large transformation strain involved in phase transformation is suppressed from occurring, and this is found to be very useful for improving the thermal fatigue life.

[0084] 2. Improving the Thermal Deformation Resistance

[0085] In components combined and attached in the state where elongation and contraction is constrained, for example, in exhaust system components for automobiles, in order to suppress thermal deformation caused by heating and cooling with exhaust gas, it is advantageous to improve the high temperature strength, in particular, the high temperature yield strength or the high temperature proportional limit. As shown in Table 2, other than a comparative material 4 (conventional material) containing 0.5% of Mo, a test material 3 having a Si amount of 4.4% that is increased from that of the comparative material 4, and various cast iron samples (test materials 4 to 9) containing the test material 3 and additional elements of various kinds were produced, and the mechanical properties from 20° C. (room temperature) to 900° C. were compared and evaluated. Tables 3 to 5 and FIGS. 2 to 5 show these results. TABLE 2 Sample No. and component composition (Unit: wt %) C Si Mn Cu Sn Cr V P S Mg Mo Ni Nb Al Comparative 3.34 3.8 0.24 0.01 0.00 0.03 0.00 0.05 0.004 0.10 0.42 0.00 0.00 0.02 material 4 (conventional product) Test 3.16 4.4 0.22 0.01 0.00 0.03 0.00 0.04 0.004 0.09 0.43 0.00 0.00 0.02 material 3 (4.4Si) Test 3.20 4.3 0.20 0.02 0.00 0.03 0.00 0.05 0.004 0.09 0.48 0.00 0.34 0.02 material 4 (0.4Nb) Test 3.21 4.4 0.23 0.01 0.00 0.04 0.28 0.05 0.004 0.10 0.45 0.00 0.04 0.02 material 5 (0.3V) Test 3.25 4.27 0.19 0.05 0.005 0.04 0.50 0.99 material 6 (1.0Ni) Test 3.21 4.32 0.20 0.20 0.05 0.005 0.05 0.51 material 7 (0.2Cu) Test 3.37 4.35 0.21 0.05 0.005 0.05 0.49 0.40 material 8 (0.4Al) Test 3.25 4.23 0.48 0.05 0.008 0.05 0.49 material 9 (0.5Mn)

[0086] TABLE 3 Temperature and tensile strength of sample (Unit: MPa) No. Test Test Test Test Test Test Test Temperature Com. material 3 material 4 material 5 material 6 material 7 material 8 material 9 (° C.) material 4 4.4Si 0.4% Nb 0.3% V 1.0% Ni 0.2% Cu 0.4% Al 0.5% Mn Tensile 20 580 640 652 681 619 655 656 641 strength 400 483 526 535 555 572 531 545 520 500 350 379 377 399 406 384 396 383 600 196 204 203 214 214 206 206 193 700 92 89 92 106 89 83 90 93 800 48 45 48 55 44 42 47 45 850 41 35 35 44 45 34 35 46 900 56 45 46 55 56

[0087] TABLE 4 Temperature and proportional limit of sample (Unit: MPa) Test Test Test Test Test Test Test No. Com. material 3 material 4 material 5 material 6 material 7 material 8 material 9 Temperature(° C.) material 4 4.4Si 0.4% Nb 0.3% V 1.0% Ni 0.2% Cu 0.4% Al 0.5% Mn Proportional 20 392 475 465 504 483 483 492 483 limit 400 312 345 359 370 363 344 363 331 500 277 311 318 342 319 312 318 312 600 144 179 168 179 178 182 182 166 700 67 64 70 77 67 61 67 70 800 31 30 32 42 34 34 40 36 850 30 25 25 33 31 25 24 31 900 41 31 33 50 50

[0088] TABLE 5 Temperature and elongation of sample (Unit: %) Test Test Test Test Test Test Test No. Com. material 3 material 4 material 5 material 6 material 7 material 8 material 9 Temperature(° C.) material 4 4.4Si 0.4% Nb 0.3% V 1.0% Ni 0.2% Cu 0.4% Al 0.5% Mn Elongation 20 17 14.5 11 9.3 1.8 13.3 12.3 13.7 400 16.6 14 13.4 14.6 8.1 10.8 10.3 11.8 500 23.7 17.4 26.9 20.1 14.8 15.4 14.5 15.4 600 16.6 27.1 37.8 26.4 23.3 26.1 25.6 26.7 700 25.3 22.9 27.9 34 34.6 38.4 32.8 33.4 800 34.3 30.3 26.3 24.2 48.7 49.3 50 51.4 850 60 36 44 30 42.5 82.5 78 54 900 75.7 49.9 50.5 78.1 76.2

[0089] These tables and graphs indicate that it is advantageous to add V, Mn, and Ni to the high Si and Mo-based cast iron material in order to improve the high temperature strength.

[0090] Then, a change in the mechanical properties between room temperature (20° C.) to 900° C. depending on the composition ratio of the additional elements was investigated. Tables 6 to 9 and FIGS. 6 to 8 show these results. TABLE 6 Samples subjected to additional addition tests (Unit: wt %) C Si Mn P S Mg Mo Ni V Test 3.25 4.28 0.22 0.048 0.005 0.048 0.49 1.58 material 10 Test 3.30 4.27 0.20 0.045 0.005 0.047 0.49 2.12 material 11 Test 3.26 4.25 0.97 0.047 0.006 0.044 0.49 material 12 Test 3.34 4.31 1.46 0.099 0.007 0.043 0.48 material 13 Test 3.25 4.33 0.20 0.050 0.006 0.046 0.48 0.67 material 14 Test 3.28 4.17 0.19 0.048 0.005 0.086 0.56 0.96 material 15

[0091] TABLE 7 Change of mechanical properties by addition of Ni Test Test Test Test Com. Test material material Com. Test material material material 4 material 6 10 11 material 4 material 6 10 11 0.0% 0.99% 1.58% 2.12% 0.0% 0.99% 1.58% 2.12% Tensile 20 640 619 627 653 elongation 20 14.5 1.8 2.0 1.2 strength 400 526 572 (%) 400 14.0 8.1 (MPa) 500 379 406 500 17.4 14.8 600 204 214 600 27.1 23.3 700 89 89 102 92 700 22.9 34.6 27.9 34.0 750 63 68 750 38.8 36.0 800 45 44 45 46 800 30.3 48.7 46.4 44.4 850 35 45 50 56 850 36.0 42.5 48.3 50.8 900 45 55 55 57 900 75.7 78.1 65.6 59.4

[0092] TABLE 8 Change of mechanical properties by addition of Mn Test Test Test Test Com. Test material material Com. Test material material material 4 material 9 12 13 material 4 material 9 12 13 0.0% 0.48% 0.97% 1.46% 0.0% 0.48% 0.97% 1.46% Tensile 20 641 625 615 elongation 20 14.5 13.7 1.7 0.9 strength 400 520 (%) 400 14.0 11.8 (MPa) 500 383 500 17.4 15.4 600 193 600 27.1 26.7 700 93 105 120 700 22.9 33.4 30.8 27.0 750 65 72 750 39.1 36.6 800 45 49 51 800 30.3 51.4 48.4 75.4 850 46 48 61 850 36.0 54.0 57.4 55.4 900 56 57 61 900 75.7 76.2 81.1 63.0

[0093] TABLE 9 Change of mechanical properties by addition of V Test Test Test Test Com. Test material material Com. Test material material material 4 material 5 14 15 material 4 material 5 14 15 0.0% 0.27% 0.67% 0.96% 0.0% 0.27% 0.67% 0.96% Tensile 20 640 681 elongation 20 14.5 9.3 strength 400 526 555 (%) 400 14.0 14.6 (MPa) 500 379 399 500 17.4 20.1 600 204 214 600 27.1 26.4 700 89 106 105 109 700 22.9 34.0 20 23.0 750 75 75 750 24.6 26 800 45 55 56 58 800 30.3 24.2 25.5 26.3 850 35 44 46 48 850 36.0 30.0 41.7 46 900 45 46 53 55 900 75.7 50.5 68.8 76.4

[0094] These results indicate that as the amount of Mn or Ni is larger, the tensile strength tends to be larger even in the vicinity of about 850° C., which is the upper limit of the metal temperature to which an exhaust system component is subjected. On the other hand, the addition of 0.1% of V can provide an effect, but even if V is added in an amount of 0.3% or more, there is no large change.

[0095] However, in the test material 13 (1.46% of Mn) and the test material 15 (0.96% of V), the pearlite ratio in the texture is increased, which leads to the assumption that elongation is reduced in a low temperature range and a middle temperature range. Therefore, it is found that addition of Mn and V in amounts of more than those is not appropriate.

[0096] The transformation temperatures were measured for the comparative material 4 and the materials 3, 5, 6 and 9 of the present invention. Table 10 shows the results. TABLE 10 Transformation temperature of major samples (Unit: ° C.) A1(A_(c1), A_(r1) A_(c1) A_(r1) average) No. No. No. 1, 2 1, 2 1, 2 Material No. Start End average average Start End average average average Com. Conventional 1 848.0 895.2 872 872 838.8 789.8 814 814 843 843 material 4 product 2 847.2 899.1 873 834.8 790.8 813 843 Test 4.4Si 1 890.8 929.7 910 907 861.7 819.6 841 841 875 874 material 3 2 882.9 925.3 904 864.1 818.5 841 873 Test 0.3V 1 888.3 921.5 905 902 858.7 814.8 837 836 871 869 material 5 2 876.6 921.5 899 858.4 813.5 836 868 Test 1.0Ni 1 871.3 914.5 893 892 842.3 795.7 819 818 856 855 material 6 2 868.8 911.9 890 838.0 797.8 818 854 Test 0.5Mn 1 877.3 917.1 897 893 847.9 806.3 827 826 862 860 material 9 2 860.1 919.1 890 845.3 805.4 825 857

[0097] Thus, in the test material 6 to which Ni was added, a reduction of the A₁ transformation point was seen, and it was found that the test material 6 was not suitable for a material of an exhaust system component. This is because Ni is an element for stabilizing austenite and reduces the Al transformation point to the low temperature side. Therefore, it was judged that an increase in the strength that was seen from the vicinity of 850° C. in the test material 6 was due to the strength of the texture that already had been transformed to austenite, and thereafter evaluation of the materials containing Ni as an additional element was omitted.

[0098] The evaluation indicates that it is advantageous to add V and Mn in order to improve the high temperature properties. Then, an effect of multiple addition of V and Mn was investigated. Tables 11 to 13 and FIGS. 9 to 11 show the results. TABLE 11 Chemical component of samples with which multiple addition effect (Unit: wt %) C Si Mn P S Mg Mo V Test 3.30 4.32 0.48 0.046 0.007 0.050 0.48 0.30 material 16 Test 3.34 4.31 1.46 0.048 0.007 0.051 0.50 0.31 material 17

[0099] TABLE 12 Mechanical properties of samples to which V + Mn are added Com. mate- Test Test Test rial 4 mate- mate- mate- Test Test conven- rial rial rial material material tional 3 5 9 16 17 product 4.4 Si 0.3 V 0.5 Mn 0.3% V + 0.5% Mn 0.3% V + 1.5% Mn Tensile strength (MPa)  20 580 640 681 641 631 610 400 483 526 555 520 551 642 500 350 379 399 383 394 503 600 196 204 214 193 219 289 700 92 89 106 93 112 157 800 48 45 55 45 55 62 850 41 35 44 46 48 66 900 56 45 46 56 56 65 Elongation (%)  20 17.0 14.5 9.3 13.7 2.7 0.3 400 16.6 14.0 14.6 11.8 9.9 3.3 500 23.7 17.4 20.1 15.4 16.5 9.7 600 16.6 27.1 26.4 26.7 20.8 16.3 700 25.3 22.9 34.0 33.4 26.8 21.7 800 34.3 30.3 24.2 51.4 53.2 46.4 850 60.0 36.0 30.0 54.0 51.5 63.3 900 75.7 49.9 50.5 76.2 88.7 67.3

[0100] TABLE 13 Hardness average Pearlite ratio Graphite area ratio Test material 16 235 HV 10% 13% Test material 17 247 HV 40% 11%

[0101] The results indicate that a larger effect is provided when V and Mn are added at the same time in combination than is added alone. The test material 17 has high hardness and a high pearlite ratio, so that elongation is low at room temperature. However, since a reduction in the elongation in a middle temperature range of interest cannot be seen, the test material 17 can be used for an exhaust system component. Furthermore, if further elongation is necessary, annealing heat treatment can be performed to degrade pearlite. In all the examples described above, there was no observation that spheroidization was inhibited by additional elements.

[0102] 3. Improving Thermal Fatigue Resistance

[0103] In the high Si spheroidal graphite cast iron containing Mo, the following two approaches are conceivable in order to enhance the thermal fatigue resistance: an approach for canceling reduction in elongation that occurs in the vicinity of middle temperature (400 to 500° C.) inherent to this material; and an approach for improving the tensile strength or the yield strength from room temperature to high temperature.

[0104] The present invention is based on the latter approach. More specifically, the present invention focuses on suppressing plastic deformation with respect to tensile strain generated in heating and cooling cycles by enhancing the yield strength (yield point or proportional limit) so as to increase the life, which is a period up to the time an initial crack occurs.

[0105] The thermal fatigue tests were conducted between 200 to 850° C. at a constraint ratio of 50%. Table 14 shows the results. TABLE 14 Thermal fatigue life and oxidation resistance of test product thermal oxidation resistance fatigue 850° C. × 100 h life oxidation oxidation thickness 200-850° C. increase decrease reduction constraint amount amount ratio ratio: 50% (mg/cm²) (mg/cm²) (%) com. conventional 263 −3.99 62.44 3.4 material material  4 test High Si + 237 −3.17 72.67 3.4 material Mo  3 test (test 293 −4.56 65.22 3.8 material material 3) +  4 0.3 V test (test 277 −3.5 55.33 3.2 material material 3) +  9 0.5 Mn test (test material material 3) + 13 1.5 Mn test (test material material 3) + 16 0.3 V + 0.5 Mn test (test 384 material material 3) + 17 0.3 V + 1.5 Mn

[0106] Table 14 clearly indicates that the material containing V, the material containing Mn, and the material containing V and Mn in which the tensile strength and the proportional limit are improved have a better thermal fatigue life than that of the comparative material 4, which is a conventional material.

[0107] Furthermore, in order to improve or stabilize the thermal fatigue characteristics of the spheroidal graphite cast iron of the present invention, in-depth research was conducted to ensure elongation from room temperature to middle temperature. When elongation is small, the thermal fatigue life is reduced, as described above. Then, as a result of examining means for ensuring elongation without reducing the total amount or the composition ratio of V and Mn, it was found that elongation of the high Si spheroidal graphite cast iron containing Mo depends significantly on the mixing ratio of C and Si, more specifically, the Si/CE value (or C/CE value).

[0108] In other words, as shown in Table 15 and FIG. 12, the elongation of the high Si spheroidal graphite cast iron was reduced drastically when the Si/CE value was 0.97 or more. TABLE 15 Relationship between Si/CE value and elongation CE Elonga- c Si Mn Cu Sn P S Mg Mo value Si/CE tion (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) value (%) Test 3.59 4.0 0.28 0.20 0.00 0.02 0.005 0.02 0.48 4.92 0.81 15.3 mate- rial 18 Test 3.24 4.0 0.36 0.07 0.00 0.02 0.007 0.02 0.53 4.58 0.88 16.4 mate- rial 19 Test 3.45 3.3 0.23 0.02 0.00 0.04 0.009 0.03 0.43 4.55 0.73 19.4 mate- rial 20 Test 3.21 4.1 0.3 0.20 0.01 0.02 0.008 0.05 0.50 4.58 0.90 15.1 mate- rial 21 Test 3.7 4.1 0.27 0.22 0.01 0.02 0.004 0.02 0.51 5.07 0.81 15.9 mate- rial 22 Test 3.05 4.7 0.28 0.20 0.01 0.02 0.004 0.03 0.54 4.62 1.02 4.3 mate- rial 23 Test 3.13 4.5 0.28 0.20 0.01 0.03 0.004 0.02 0.75 4.63 0.97 5.3 mate- rial 24 Test 3.33 4.0 0.28 0.22 0.00 0.02 0.005 0.04 0.32 4.66 0.86 17.7 mate- rial 25 Test 3.21 4.5 0.27 0.20 0.00 0.02 0.005 0.03 0.65 4.71 0.96 14.2 mate- rial 26 Test 3.20 4.2 0.29 0.19 0.02 0.02 0.04 0.03 0.54 4.60 0.91 14.4 mate- rial 27

[0109] 4. Improving the Oxidation Resistance

[0110] As shown in Table 14, the results of an oxidation test in which samples were held in air at 850° C. for 100 hours indicate that the material containing V, the material containing Mn and the material containing V and Mn were substantially equal to the conventional material (comparative material 4) in the decrease amount of oxidation, the increase amount of oxidation, and the reduction ratio of thickness, and it was found that the oxidation resistance rather depends on the Si content.

[0111] The present invention is not limited to the above-described embodiments of the present invention, and can be changed or modified based on the technical idea of the present invention. Hereinafter, variations of the application in which the spheroidal graphite cast iron is used for an exhaust system component of the present invention will be described briefly.

[0112] Even one exhaust system component has a large thermal load in some places and a small thermal load in other places, and in some places, thermal expansion cannot be allowed, and in other places, thermal expansion can be allowed. Therefore, when the spheroidal graphite cast iron of the present invention is used in places having a large thermal load because welding or mechanical connection is performed or places where thermal expansion cannot be allowed, the heat resistance of the exhaust system component can be improved. Furthermore, an exhaust manifold and a turbo housing or a turbo housing-integrated exhaust manifold are cast-molded to improve the heat resistance and reduce the component cost. Furthermore, the spheroidal graphite cast iron of the present invention has not only a better heat resistance, but also compression strength that is inherent to cast iron, so that the present invention also can be applied to a substalk material by aluminum-low pressure cast that requires oxidation resistance and compression deformation resistance at high temperature.

[0113] The ferrite-based spheroidal graphite cast iron of the present invention has excellent tensile strength and yield strength in a range from 20° C., which is room temperature, to high temperature (around 800 to 900° C.). Therefore, when this spheroidal graphite cast iron is applied to an exhaust system component, for example, an exhaust manifold, the component can withstand high temperature exhaust gas sufficiently, and therefore the temperature of the exhaust gas can be increased, and efficient purification of the exhaust gas and fuel saving can be achieved. 

What is claimed is:
 1. A ferrite-based spheroidal graphite cast iron comprising C, Si, Mo, V, Mn, and Mg, wherein a remaining portion is composed of Fe and inevitable impurities.
 2. The ferrite-based spheroidal graphite cast iron according to claim 1, wherein contents of the elements in % by weight are as follows: C: 3.1 to 4.0%; Si: 3.6 to 4.6%; Mo: 0.3 to 1.0%; V: 0.1 to 1.0%; Mn: 0.15 to 1.6%; and Mg: 0.02 to 0.10%.
 3. The ferrite-based spheroidal graphite cast iron according to claim 1 or 2, wherein a total content of V and Mn is 0.3 to 2.0 wt %.
 4. The ferrite-based spheroidal graphite cast iron according to any one of claims 1 to 3, wherein a Si/CE value is 0.97 or less.
 5. An exhaust system component produced using the ferrite-based spheroidal graphite cast iron according to any one of claims 1 to
 4. 6. The exhaust system component according to claim 5, wherein the exhaust system component is an exhaust manifold, a turbo housing, a turbo housing-integrated exhaust manifold, or a turbo outlet pipe. 